This invention relates generally to improvements in the performance of high-temperature superconducting bearings. More particularly, this invention relates to improvements in the levitation pressure and reductions in friction losses in superconducting bearings.
One type of superconducting thrust bearing comprises a double-ring permanent magnet that is levitated over an array of high-temperature superconducting elements. The rotating part of such a bearing consists of an outer permanent magnet ring and an inner permanent magnet ring which are connected by a ferromagnetic annular disc. This disc serves as a low reluctance path along the top of the ring pair. Both of these rings are magnetized with a vertical polarization and in opposite directions. The stator part of the bearing consists of an array of high-temperature superconducting elements. These elements can be formed in a variety of shapes. For example, this array can be composed of a set of three annular rings with each of the rings comprised of individual high-temperature superconducting elements that are located circumferentially along each ring. In one arrangement, all of the high-temperature superconducting elements would have the same crystalline orientation with the c-axis of each element being vertical.
Such array arrangements, although reasonably effective, also result in some velocity dependent loss in the bearing. In order to produce levitation, high-temperature superconducting elements must be magnetized. Because these elements are discrete and are magnetized by macroscopic circulating currents, they are most strongly magnetized towards the center of each element. As the permanent magnet rotates above the array, the magnetized high-temperature superconducting elements produce an effective alternating current magnetic field over the permanent magnet which causes eddy currents to flow in the permanent magnet contributing to rotational loss. This loss will be proportional to the rotational velocity.
Ideally, one would like to use a monolithic block of a high-temperature superconductor with shielding currents to improve the levitation pressure and eliminate the eddy current production in the permanent magnets. There have been several attempts to achieve such an ideal situation but all have their drawbacks. A sintered high-temperature superconductor could be made of large enough size to establish the electrical current pattern for the ideal situation. Unfortunately, however sintered high-temperature superconductors have a relatively small critical-current density and do not produce a large levitation force. Melt-textured high-temperature superconductors are often used to obtain a large levitation force, but they cannot be economically grown to a sufficiently large size desired for the permanent magnet; and techniques to join elements together have not been developed thus far.
As a consequence of these restrictions, high-temperature superconducting elements must be arranged in an array. When in an array, the tiling arrangement of the high-temperature superconductor rings is critical for the levitation forces generated. Each individual tile has an electrical current running inside it in a loop. The axis about which this current loop flows, or c-axis, is critical for forming an effective arrangement. For a double-ring permanent magnet system, one possible tiling arrangement has the c-axis of each element oriented vertically. In such an arrangement, the elements in the center ring will have two current loops that flow in opposite directions as a result of the magnetic fields exerted on the center ring elements by the two permanent magnets. Under this arrangement, however, the elements are less effective levitators because adjacent currents will flow in opposite directions along the edge of adjacent elements and essentially partially offset each other in terms of levitation force. Thus this portion of the high-temperature superconducting element is not very useful for levitation. Additionally, these currents result in a decrease in the magnetic field immediately above that element edge. This causes the alternating current component of the magnetic field and a velocity dependent loss in the bearing rotation due to the eddy currents induced in the permanent magnet.